1. Field of the Invention
The invention relates to a bandgap circuit and a start circuit thereof. Particularly, the invention relates to a bandgap circuit having a start function and a start circuit thereof.
2. Description of Related Art
FIG. 1 is a circuit diagram of a conventional bandgap circuit. As shown in FIG. 1, the bandgap circuit 100 includes a start circuit 110 and a reference current generating circuit 120. The reference current generating circuit 120 includes a plurality of current mirrors 121-124, and the current mirrors 121-124 are connected in cascade with each other and have bias nodes N11-N14. Moreover, the current mirrors 121-124 are electrically connected to ground through bipolar transistors BT11-BT12 and a resistor R1. In this way, the reference current generating circuit 120 can map a reference current IR1 proportional to absolute temperature (PTAT) through P-channel transistors MT11 and MT12.
In order to ensure the reference current generating circuit 120 to normally provide the reference current IR1, the start circuit 110 is used to break the reference current generating circuit 120 away from a zero-current state. In operation, a control end of a switch SW11 receives a node voltage VN1 from the bias node N14, and a switch SW12 determines whether or not to provide a start voltage VT1 to the bias node N12 according to its conducting state. Where, during an initial stage that a power voltage VD1 is gradually increased from a ground voltage, the node voltage VN1 is close to the ground voltage, so that the switch SW11 cannot be turned on.
Now, the switch SW12 receives the power voltage VD1 through a load unit 111, and conducts two ends thereof to provide the start voltage VT1 to the bias node N12. In this way, the reference current generating circuit 120 can leave the zero-current state according to the start voltage VT1. Then, the node voltage VN1 is gradually increased as the power voltage VD1 is increased, so as to turn on the switch SW11. Now, the switch SW12 receives the ground voltage and cannot conduct the two ends thereof. In this way, the start circuit 110 stops outputting the start voltage VT1, and the reference voltage generating circuit 120 can normally supply the reference current IR1.
However, when the power-on/off time of the system is excessively short, i.e. the power voltage VD1 is quickly switched, residual charges are accumulated at the bias nodes N11-N14 in a large amount. In this way, during an initial stage of powering on the system, the switch SW11 cannot be normally turned off, and accordingly the switch SW12 cannot be normally turned on. In other words, when the power voltage VD1 is quickly switched, the start circuit 110 cannot normally operate, which may lead to a result that the reference current generating circuit 120 cannot leave the zero-current state.